Plant Doctor Archive

Rusty leaves

CAN you help with my black hollyhock? The leaves keep getting rust on their undersides. I've changed the potting mix twice and sprayed them, but have had no luck controlling it.

THERE are hundreds of different fungi causing diseases on plants that are collectively known as rusts. Each rust generally attacks only a narrow range of plants. For example, fuchsia rust doesn't affect peach trees and vice versa.

Some rusts are relatively easy to control, others very difficult, especially if the host plant (such as your hollyhock) is susceptible to the disease. Changing the potting mix won't help, as spores causing the disease just drift in from all around the neighbourhood.

Apart from getting rid of the plant altogether there are several steps you can take to try to minimise the problem, though you'll probably always have it to some extent. In autumn, once all the leaves and stems have started to die back, prune them off close to the ground and destroy them - either put them out in the rubbish or burn them. Rake up all dead leaves from around the plant. In spring, fertilise and mulch to encourage strong new growth, as weak shoots are more likely to become diseased. Try to make sure there's plenty of air movement around the plant. Prune nearby plants back if necessary.

Before the plant becomes diseased, spray with a protectant fungicide such as Bravo or Fungus & Mildew Spray. Once symptoms show, try a spray such as Fungus Fighter or Greenguard. You'll have to spray regularly, according to the label recommendations, for long-term control. Don't just use one product all the time - alternate between two or three of the above for best overall effect. And ideally pick off any affected leaves as soon as they appear to prevent even more spores being released to cause further infection.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.